1. Field of the Invention
The present invention relates to an electromagnetic valve (which may also be referred to as the solenoid valve or magnet valve) including a valve element which is driven to an opened or closed state by means of a plunger under the influence of a magnetic field generated by a coil upon electric energization thereof. More specifically, the present invention is concerned with an electromagnetic valve of a duty control type for controlling an amount or flow rate of a fluid flowing or passing through the valve by changing periodically a time period during which the coil is electrically energized.
2. Description of Related Art
In the electromagnetic valve including a valve element driven to an opened or closed state by means of a plunger under the influence of a magnetic field generated by a coil upon electrical energization thereof, there may arise such a situation in which the plunger continues to remain in the state contacting with a fixed core of the electromagnet due to an excessively large residual magnetic force even after the electric energization of the coil has been broken. As an attempt for avoiding such undesirable situation, there has already been proposed such a valve structure in which a spacer of a non-magnetic material is interposed between the plunger and the fixed core. For more particulars, reference may have to be made to, for example, Japanese Utility Model Registration Publication No. 38779/1995.
In the hitherto known electromagnetic valve of the structure such as mentioned above, the surface of the spacer which faces oppositely to the plunger is formed flatly. Consequently, the areas over which the spacer is brought into contact with the plunger and the fixed core, respectively, are large. In the spacer of this type, intimacy or affinity, so to say, will occur in the spacer portion which is brought into contact with the plunger, being accompanied with increase of the contact area, due to deformation and abrasion of the spacer brought about by repetitive application of an impulsive force when the electromagnetic valve is operated over an extended time. When such affinity becomes remarkable, cohesion taking place between the contacting surfaces of the spacer and the plunger increases particularly at a low temperature at which the viscosity of the fluid under control is large. The increase of the cohesion in turn involves a lag in the response behavior of the electromagnetic valve in particular upon interruption of the electrical energization, giving rise to a problem.
This problem is serious particularly in the electromagnetic valve of a duty control type in which the amount or flow rate of a fluid flowing through the valve is controlled by changing the timings at which the valve element is driven to opened and closed states by changing the time durations of electric energization and deenergization of the coil repeated periodically, because the control function or performance of the duty control type electromagnetic valve may then be degraded considerably. Furthermore, increasing of the contact area between the spacer and the plunger tends to increase the operation noise of the electromagnetic valve, giving rise to another problem.